Fuel cells are electrochemical devices which can convert energy stored in fuels to electrical energy with high efficiencies. High temperature fuel cells include solid oxide and molten carbonate fuel cells. These fuel cells may operate using hydrogen and/or hydrocarbon fuels. There are classes of fuel cells, such as the solid oxide reversible fuel cells, that also allow reversed operation, such that water or other oxidized fuel can be reduced to unoxidized fuel using electrical energy as an input.
In a high temperature fuel cell system such as a solid oxide fuel cell (SOFC) system, an oxidizing flow is passed through the cathode side of the fuel cell while a fuel flow is passed through the anode side of the fuel cell. The oxidizing flow is typically air, while the fuel flow is typically a hydrogen-rich gas created by reforming a hydrocarbon fuel source. The fuel cell, typically operating at a temperature between 750° C. and 950° C., enables the transport of negatively charged oxygen ions from the cathode flow stream to the anode flow stream, where the ions combine with either free hydrogen or hydrogen in a hydrocarbon molecule to form water vapor and/or with carbon monoxide to form carbon dioxide. The excess electrons from the negatively charged ions are routed back to the cathode side of the fuel cell through an electrical circuit completed between anode and cathode, resulting in an electrical current flow through the circuit.
Fuel cell stacks are frequently built from a multiplicity of cells in the form of planar elements, tubes, or other geometries. Fuel cell stacks, particularly those with planar geometry, often use seals between electrolyte and interconnect surfaces to contain fuel and air and at various locations within the stack. While it is desirable for seals to be effective at start up temperatures to prevent escape (and potential ignition) of fuel gasses, these seals must maintain their operating integrity at high operating temperatures and an oxidizing, reducing, or mixed (i.e., oxidizing on one side on one side of the seal and reducing on the other) environment. Expansion and contraction of fuel cell stack components (including seals) due to thermal cycling or compression should not result in damage to of any of the components during a seal's expected life.
Many types of seals used at elevated temperatures, such as brazes and metal gaskets, often have a limited life, tolerating only a relatively few number of thermal cycles before they fail due to differences in the coefficients of thermal expansion that result in mechanical stresses that can lead to failure of the seal or the components of the assembly. Some assemblies are difficult to seal with brazes or gaskets because of operating conditions or material incompatibilities. Also, brazes and metal gaskets often present difficulties and high costs of fabrication and assembly due to the tighter tolerances which are required, in flatness for example.
Many compliant seals, such as elastomeric O-rings and gaskets, form effective seals at start up temperatures, do not crack and tend to absorb stresses in an assembly that arise from thermal expansion and compression. However, these seals cannot be used in high temperature conditions because the elastomeric materials used in them decompose, degrade, or oxidize at high temperatures.
Glass and glass ceramic compounds have been shown to be able to provide robust high temperature seals. However, they have a major shortcoming in that they are ineffective at forming a hermetic seal until the stack reaches their softening temperature. Selection of the glass or glass ceramic composition inherently defines the melting characteristics and viscosity of the seal as a function of temperature. Thus, the temperature at which an effective seal is initially able to form can be tailored based on composition. However, if a glass or glass ceramic is used with a low enough softening temperature that an effective seal is able to form at relatively low temperatures, the viscosity of the glass at typical SOFC operating conditions may be low enough that the system pressure can push the seal out of position. Conversely, if a glass or glass ceramic is used with a high enough softening temperature that an effective seal is able to be maintained at typical SOFC operating conditions, the seal's relatively high softening temperature prevents it from forming an effective seal early in the SOFC's heating cycle.
Failure to establish and maintain an effective seal during the heating cycle of a SOFC allows the fuel gasses to escape. These escaping gasses can ignite causing local heating and potentially changing the composition and/or properties of the sealing materials used.